Chapter 39. How Multicast IP and the MBone Work

Today's Internet is made up not just of text, but of sound, video, animation, 3D objects, and more. Web pages are interactive, and video-based shows are commonly broadcast. Already, in fact, video-based shows and movies are being broadcast.

This can cause serious congestion on the wires and networks that make up the Internet. That's only one problem with video broadcasting over the Internet. There is another problem as wellthere's no practical way for broadcasts to be sent over the Internet because the files clog it up. Suppose, for example, that someone wants to broadcast a telecast of a concert. The size of the file containing that broadcast might be 50 megabytes (MB). Now imagine that 10,000 people want to watch the concert. That 50MB file needs to be sent individually to each of those 10,000 people. As you can imagine, that single broadcast could easily clog entire sections of the Internet, which would prevent the broadcast from being delivered.

One answer is the Multicast Backbone, or the MBone. The MBone is a high-capacity Internet backbone for transmitting broadcasts using the IP multicast protocol. The MBone enables broadcasts to start out as a single transmission instead of, for example, 10,000 transmissions. Inside that single transmission are the addresses of all the people who want to see the broadcast. As the file is sent across the Internet, it eventually makes copies of itself when necessary and delivers the broadcast to the networks and individuals who want to see it.

Suppose that 100 people want to see a broadcast of a 50MB file. Fifty people who want to see it are connected to the Internet via the WorldNet Internet service provider, 25 people are on a corporate network at zd.com, and another 25 use the Internet Access Company Internet service provider. When the broadcast goes out, it goes out as one single file, not 100 separate files. The file then splits into three parts: One part goes to WorldNet, one part goes to zd.com, and one part goes to the Internet Access Company. After the file is on each of those separate networks, it is delivered to the people inside the networks who want to see it. The key here, however, is that instead of 100 files of 50MB traveling across the Internet5 gigabytes (GB) of dataonly three 50MB files travel, or 150MB of data. As you can see, the MBone can cut down tremendously the amount of traffic traveling across the Internet.